Reaction mechanism and product branching ratios of the CH + C3H4 reactions: a theoretical study

Abstract

Two ground-state CH radical reactions with the C₃H₄ isomers allene and methylacetylene occurring along the C₄H₅ potential energy surface (PES) were studied to probe the reaction mechanisms and final product distributions. The calculations were performed using a CCSD(T)-F12//B2PLYPD3 PES in combination with the 1-D chemical master equation. The reaction between the CH radical and allene was found to lead to exclusive “funneling” of the energized C₄H₅ intermediates into linear C₄H₅ configurations before reaching the exit channels, regardless of the specific nature of the initial bimolecular reactive encounter. In the case of the CH radical reaction with methylacetylene, energized C₄H₅ three-membered ring structures underwent H loss in significant amounts resulting in the production of a cyclic C₄H₄ methylenecyclopropene product, in accordance with experiments. The theoretical product distribution at room temperature for methylacetylene + CH was ∼35% methylenecyclopropene, ∼36% vinylacetylene, and ∼28% 1,2,3-butatriene, which is in agreement with the available experimental data. The distribution for allene + CH was ∼93% vinylacetylene, ∼4% 1,2,3-butatriene and ∼3% acetylene + vinyl, which overestimates the experimental yield of vinylacetylene and underestimates that of 1,2,3-butatriene by ∼10%. The possible reasons for this slight quantitative deviation of the theoretical results obtained within statistical treatment from the experiment are discussed.